Guozhu Li

Catalytic hydrogenation by noble-metal nanocrystals with well-defined facets: a review

Recent studies with better catalytic models show the structure sensitivity of noble-metals for catalytic hydrogenation. The size and shape of noble-metal nanocrystals have a great impact on their reaction performance in hydrogenation. Essentially, the exposed crystal planes, which primarily determine the morphology of a nanocrystal, tremendously affect its catalytic behavior. Therefore, many new methods involving controllable nucleation and growth processes have been developed to prepare uniform noble-metal nanocrystals with tunable sizes and shapes for catalytic hydrogenation. This paper presents a brief overview of the activity and selectivity of noble-metal nanocrystals with well-defined facets in the field of catalytic hydrogenation. High activity and controllable selectivity in hydrogenation have been achieved based on the shaped noble-metal nanocatalysts.

Catalytic activity of shaped platinum nanoparticles for hydrogenation: a kinetic study

Aqueous-phase hydrogenation of 4-nitrophenol (4-NP) was investigated in the presence of free-standing or supported Pt nanoparticles (NPs) of various shapes. Uniform cubic and pseudo-tetrahedral Pt NPs with sub-10 nm sizes were obtained by the direction of bio-inspired small molecules, 3-hydroxybutyric acid (3-HB) and tropic acid (TA), respectively. The catalytic activity is found to be strongly affected by the nanoparticle shape and the support, SBA-15. Pseudo-tetrahedral Pt NPs directed by TA have superior activity to the cubic ones with 3-HB. But when loaded on SBA-15, cubic Pt(3-HB) NPs showed better performance than Pt(TA) NPs. Then the experimental data are fitted to a Langmuir–Hinshelwood (L–H) model involving a surface reaction controlling mechanism for 4-NP hydrogenation. This model follows a dual-site adsorption with molecular adsorption of 4-NP and dissociative adsorption of hydrogen. The results show that the reaction catalyzed by pseudo-tetrahedral Pt(TA) NPs has lower apparent activation energy than that by the cubic Pt(3-HB) NPs. But after loading these NPs onto SBA-15, the apparent activation energy of Pt(3-HB)@SBA-15 decreased while that of Pt(TA)@SBA-15 increased in comparison with their corresponding free-standing NPs.

Nanocomposites of polymer brush and inorganic nanoparticles: preparation, characterization and application

The formation of nanocomposites by embedding inorganic nanoparticles (NPs) in polymer brushes has been studied extensively for the development of functional surfaces. Polymer brushes are particularly useful matrices for the preparation of nanocomposites, because the macromolecular matrix acts as a reaction chamber for nanoparticle synthesis, as a scaffold for immobilization, and as a capping agent for preventing nanoparticle aggregation. Moreover, synergies between the polymer chains and the inorganic NPs will grant the composite new properties. The stimuli-responsive polymers/NPs endow the composites with excellent properties for many applications, such as in sensors, detectors, and electronic/optical devices. We tackle in this review the use of a subset of polymer brushes (e.g., polyelectrolytes and polyampholytes) for the embedment of inorganic NPs to make composite surfaces/NPs with specific functions.

Water-dispersible Fe3O4 nanowires as efficient supports for noble-metal catalysed aqueous reactions

Water-dispersible magnetic Fe3O4 nanowires were synthesized at room temperature by the coprecipitation method using bio-inspired dopamine as a shape-directing surfactant. The as-synthesized nanowires were used to load a noble metal (Pd or Pt) for the preparation of magnetic nanocatalysts. The Fe3O4-nanowire supported noble metal exhibits bi-functional properties with stable water dispersion and excellent catalytic activity toward the hydrogenation of 4-nitrophenol and reduction of 4-nitrophenol by NaBH4 in water. In addition, the magnetic heterostructured nanocatalysts show good separation ability and reusability for at least 5 successive cycles. Moreover, Pd/Fe3O4 nanowires can also act as an efficient catalyst for the Suzuki reaction under aqueous conditions.

Surface-Engineered Polydopamine Particles as an Efficient Support for Catalytic Applications

Mussel-inspired polydopamine (PDA) particles with the size of ∼270 nm are used as a support of palladium (Pd) nanoparticles (NPs) for catalyst preparation. The surface morphology of the PDA particle has been modified via corrosion of CF3COOH. Surface chemistry of the obtained PDA particle has been engineered by the formation of a carboxylic acid-terminated alkanethiol monolayer. The obtained self-assembled monolayer-modified PDA (SAM-PDA) particles are used to load Pd NPs by simply adding H2PdCl4 solution to a suspension of SAM-PDA particles at room temperature. Transmission electron microscopy, energy-dispersive X-ray mapping, dynamic light scattering, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis, and Fourier transform infrared are used to characterize the catalyst and to investigate the process. Uniform Pd NPs (2-3 nm) have been well-dispersed on the SAM-PDA particles via controllable surface engineering. Surface charges and interactions with a metal ion are regulated by the monolayer of carboxylic acids. The surface chemistry of PDA particles has been finely engineered for efficient loading of noble metal NPs. The obtained Pd/SAM-PDA catalyst has shown greatly increased activity and good reusability compared with Pd/PDA in the reduction of 4-nitrophenol (4-NP) by sodium borohydride or H2. The kinetic data of 4-NP hydrogenation catalyzed by Pd/SAM-PDA are fitted to a Langmuir-Hinshelwood (L-H) model, and the calculated apparent activation energy of this process is 40.77 kJ mol-1.

Ceria-promoted Ni–Co/Al2O3 Catalysts for n-dodecane Steam Reforming

Catalytic steam reforming of long-chain hydrocarbons has drawn great attention for hydrogen production or heat removement, which can be used for on-board/on-site fuel cell or hypersonic vehicle thermal protection. In this study, a series of Ni–Co bi-metal catalysts supported on Al2O3 with different ceria loadings have been prepared by a two-step impregnation method. Physical states and chemical properties of the as-synthesized catalysts have been characterized by XRD, nitrogen adsorption–desorption, H2-TPR and TEM. Steam reforming of n-dodecane has been carried out to evaluate catalytic performance of the obtained catalysts at 700xa0°C and atmospheric pressure in a fixed-bed tubular reactor. Both effects of CeO2 addition and Ni–Co alloying have been investigated to enhance catalytic activity and stability. After optimization, the catalyst promoted by 5 wt% ceria (NC/5CeAl) exhibits the highest conversion of n-dodecane (89xa0%) and the lowest coke deposition (reduced by 50xa0% compared with the one without ceria loading). The improvement in the performance of n-dodecane steam reforming is ascribed to the reduction of Ni particle size on alumina and the acceleration of coke gasification via redox, which has been achieved by Ni–Co alloying and CeO2 promotion.Graphical AbstractCeria-promoted Ni-Co/Al2O3 catalysts have been developed and applied for n-dodecane steam reforming. The obtained results showed that ceria addition and Ni–Co alloying play an important role in n-dodecane steam reforming.

Enhancing tetralin hydrogenation activity and sulphur-tolerance of Pt/MCM-41 catalyst with Al(NO3)3, AlCl3 and Al(CH3)3

High efficiency and sulfur-tolerant Pt–Al/MCM-41 catalysts were prepared with Al(NO3)3, AlCl3 and Al(CH3)3 as promoters. The highest tetralin conversion and the best sulfur-tolerance were achieved on the Al(CH3)3-promoted catalyst. The pseudo first-order rate constants of this catalyst for tetralin hydrogenation are about 2–5 and 8–10 times as high as that of the Al-free one under sulfur-free and sulfur-containing conditions, respectively. AlCl3 also improves the catalytic activity and sulfur-tolerance considerably while Al(NO3)3 has certain promoting effects on sulfur-tolerance. These promoting effects can be ascribed to (i) the anchor and isolation effects improving the platinum dispersion which benefits tetralin hydrogenation and sulfur-tolerance. (ii) The electron-withdrawing effect is in the order Al(NO3)3 > AlCl3 > Al(CH3)3, resulting in a similarly electron deficient Ptδ+ order of the corresponding catalysts. The sulfur-tolerance is in favour of electron deficient Ptδ+, though the opposite of this is true for the tetralin hydrogenation. (iii) The acid sites and especially the hydroxyl sites, provide additional active sites and spillover hydrogen which also contribute to the tetralin hydrogenation and sulfur-tolerance of the catalysts.

AlCl3-Promoted MCM-41-Supported Platinum Catalysts with High Activity and Sulfur-Tolerance for Tetralin Hydrogenation: Effect of Al/Pt Ratio

AlCl3-promoted MCM-41-supported Pt catalysts with different Al/Pt mole ratios (0, 1, 2, 4, 8) were prepared for tetralin (THN) hydrogenation. Py-FTIR results show that the acid amount of catalysts increases with the increase of Al/Pt ratio. XRD, TEM and CO-FTIR results indicate that platinum dispersion changes with Al/Pt ratio and is highest at Al/Ptxa0=xa02. The specific rate constant of THN hydrogenation under sulfur-free condition is roughly proportional to platinum dispersion. Thermogravimetric analysis of the adsorbed THN and benzothiophene (BT) indicates that BT is more tightly adsorbed on the catalysts than THN, which is the main reason for catalytic deactivation. The introduction of aluminum enhances both THN and BT adsorptions. All Al-containing catalysts have better sulfur-tolerance than the Al-free one, which is attributed to that (1) high catalytic activity reduces the proportion of BT-deactivated catalyst; (2) Al3+ site might provide additional active sites; (3) electron-deficient Ptδ+ promotes the BT competitive hydrogenation activity.Graphical Abstractxa0

Doping carbon nanotubes with N, S, and B for electrocatalytic oxygen reduction: a systematic investigation on single, double, and triple doped modes

In this work, polydopamine (PDA)-coated multi-walled carbon nanotubes (CNTs) have been employed as reactive platforms for the anchoring of multiple heteroatom dopants. Single-, double-, and triple-doped CNTs with the elements N, S and B have been successfully prepared and systematically investigated as electrocatalysts for the oxygen reduction reaction (ORR). The obtained catalysts have been fully characterized by the nitrogen-adsorption technique, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Combining with the physicochemical properties of different doped CNTs and their catalytic performance, various doping modes are compared and the mechanism of these processes has been investigated. The synergetic effects on double and triple doped CNTs with N, S, and B are found. The new non-metal catalyst N–S–B-CNTs exhibits high electrocatalytic activity and good stability for ORR.

Density Functional Theory Analysis of Anthraquinone Derivative Hydrogenation over Palladium Catalyst

A density functional theory (DFT) analysis was conducted on the hydrogenation of 2-alkyl-anthraquinone (AQ), including 2-ethyl-9,10-anthraquinone (eAQ) and 2-ethyl-5,6,7,8-tetrahydro-9,10-anthraquinone (H4 eAQ), to the corresponding anthrahydroquinone (AQH2 ) over a Pd6 H2 cluster. Hydrogenation of H4 eAQ is suggested to be more favorable than that of eAQ owing to a higher adsorption energy of the reactant (H4 eAQ), lower barrier of activation energy, and smaller desorption energy of the target product (2-ethyl-5,6,7,8-tetrahydro-9,10-anthrahydroquinone, H4 eAQH2 ). For the most probable reaction routes, the energy barrier of the second hydrogenation step of AQ is circa 8u2005kcalu2009mol-1 higher than that of the first step. Electron transfer of these processes were systematically investigated. Facile electron transfer from Pd6 H2 cluster to AQ/AQH intermediate favors the hydrogenation of C=O. The electron delocalization over the boundary aromatic ring of AQ/AQH intermediate and the electron-withdrawing effect of C=O are responsible for the electron transfer. In addition, a pathway of the electron transfer is proposed for the adsorption and subsequent hydrogenation of AQ on the surface of Pd6 H2 cluster. The electron transfers from the abstracted H atom (reactive H) to a neighbor Pd atom (PdH ), and finally goes to the carbonyl group through the C4 atom of AQ aromatic ring (C4 ).